In the game of Go (Wei Qi/Badouk), "iron pillar" refers to two stones of the same color placed in directly adjacent points, as shown below:
...
.o.
.o.
...
Of course, it could be formed horizontally as well. The iron pillar is a very strong shape, but has no flexibility. The two stones are connected, so they must live or die together; after forming an iron pillar, you lose the choice of sacrificing one of the stones without also losing the other. Therefore, it should only be played in areas where strength is important, i.e. if there are stones of the opponent's in the immediate vicinity.
It may seem trivial and a waste of time to have a special name for something as simple as two adjacent stones, but the iron pillar is often used as part of larger, more complicated shapes. One commonly occuring one that doesn't have its own special name is ikken tobi from the iron pillar, as shown here:
......
..oo..
....a.
.cbo..
......
It is more flexible than the iron pillar, since either side can be sacrificed, but it is also effectively connected if you want it to be, since the opponent cannot cut unless he plays at the point marked a first, in which case you can simply form a bamboo joint at b or a table shape at c (see bamboo joint and table shape nodes for explanations of these shapes... it's worth noting that a bamboo joint is just two parallel iron pillars with an empty row between them).